In order to be released from their host cell, newly formed envelope viruses must bud from the plasma membrane. In examining the mechanism of budding of the retroviruses Rous sarcoma virus (RSV) and HIV, three groups have discovered a role for ubiquitin in the budding process. Ubiquitin is a small cellular protein that becomes covalently linked to other proteins and is commonly thought to function as a tag that leads to protein degradation; in some instances, ubiquitin can act as a signal for protein internalization via endocytosis.

Now it appears that ubiquitin also participates in virus budding. The Gag protein of retroviruses provides the structural components required to build virus particles, but the processes of budding and viral maturation are not completely understood. Schubert et al. and Patnaik et al. show that by reducing the levels of ubiquitin available in HIV- or RSV-infected cells, viral budding is inhibited at a late step in the process. Strack et al. show that engineered HIV and RSV Gag proteins only caused budding when they were able to recruit the enzyme ubiquitin ligase to the budding site at the host cell surface. A fourth study by Harty et al. demonstrates the ubiquitination of VP40, a protein involved in late budding stages of Ebola virus. Taken together, these papers suggest that ubiquitin helps these viruses to complete the budding process, and it is tempting to suppose that the endocytosis-promoting activity of ubiquitination has been commandeered by these viral interlopers. — SMH